The present invention relates to methods and apparatus for quantitatively measuring or characterizing the reception quality of a received frequency-modulated ultra-shortwave signal.
Because broadcast frequency-modulated ultrashortwave signals spread out in space as they are propagated, such a signal when received by a particular receiving antenna will comprise not only a directly transmitted component but also, superimposed thereon, signal components transmitted to the receiving antenna indirectly as a result of reflection of the broadcast waves off of reflecting structures, such as buildings, geological structures and the like. Those signal components attributable to such reflection of the broadcast waves can be considered as constituting a noise signal which modulates the received IF carrier with respect to both amplitude and phase, in a sense of course detracting from reception quality.
Quantitative measurement or characterization of the quality of reception in such circumstances is extremely important in many instances, for example where that information is to be relied upon for steering or adjusting the receiving antenna. Methods and systems serving this purpose are known; attention is directed, by way of example, to the British Broadcasting Corporation research report BBC RD 1975/33. With such conventional techniques, the amplitude modulation of the frequency-modulated carrier resulting from broadcast-wave reflections is ascertained as a function of frequency or frequency deviation of the carrier, and the quantitative reception quality characterization is derived from that relationship, in a manner discussed more fully below.
As already stated wave-reflection phenomena can be considered to act as a noise signal serving to modulate the frequency-modulated carrier with respect to both amplitude and phase. In general, it is not possible, i.e., during the actual reception of an ordinary ultrashort-wave FM broadcast, to continually ascertain with any meaningful precision to degree to which the carrier has become phase-modulated by the wave-reflection noise signal, because it is not possible to separately ascertain what phase variations, or components thereof, of the carrier signal are to be associated with the actual message signal, on the one hand, and the wave-reflection noise signal, on the other hand.
In contrast, it is possible to quantitatively ascertain the degree to which the frequency-modulated carrier has become amplitude-modulated by wave-reflection noise, for the simple reason that ultrashort-wave FM broadcasters of course take pains to assure that their transmitted carriers are of constant amplitude. Accordingly, at the receiver end, any amplitude fluctuations exhibited by the carrier can only be the result of noise.
In the already mentioned British Broadcasting Corporation research report BBC RD 1975/33 ("A field strength measuring receiver for band II"), a signal whose instantaneous value indicates the instantaneous amplitude of the frequency-modulated carrier is applied to the vertical-deflection input of an oscilloscope screen, and a signal whose instantaneous value indicates the instantaneous frequency or instantaneous frequency deviation is applied to the horizontal-deflection input. The resulting waveform generated on the oscilloscope screen displays the functional dependence of the frequency-modulated carrier's amplitude upon the frequency of the frequency-modulated carrier. The oscilloscope screen itself is provided with a horizontal axis directly calibrated in units of frequency. The oscilloscope screen is furthermore provided with a vertical axis directly calibrated in units of percentage amplitude modulation relative to the amplitude of the IF carrier if no wave-reflection noise phenomena were involved. E.g., when the ordinate value of the displayed graph is coincident with the horizontal axis of the display screen, this means that, here, the amplitude of the carrier equals 100% of the amplitude which the carrier would have if all wave-reflection modulating noise signals could somehow be removed. Where the ordinate value of the displayed curve is located above the horizontal axis, this means that the wave-reflection noise signals are increasing the amplitude of the IF carrier relative to such value; likewise, where the ordinate value of the displayed curve is located below the horizontal axis, this means that the wave-reflection noise signals are decreasing the amplitude of the IF carrier relative to the imaginary reflection-free situation.
Using the conventional procedure, the analyst notes the maximum ordinate value of the displayed curve and also the periodicity of the curve. After the periodicity of the curve has been perceived and its period decided upon, the analyst forms the reciprocal of the period, this constituting information concerning the delay in the reception of the reflected waves. Then, the analyst refers to a tabulation, e.g., a nomograph, which has been set up in advance to provide quantitative reception-quality values as a function of measured carrier amplitude-modulation values and reflected-wave delay times. This analytical procedure is not only laborious and time-consuming but also, to a very great extent, subjective in nature, in the sense that it very much depends upon the experience, skill and imagination of the analyst. Thus, for example, if one or more reflecting bodies are located rather close to the receiving antenna, and/or if many reflected-wave phenomena are concurrently contributing to the amplitude-versus-frequency curve generated on the display screen, the displayed information may not actually contain the requisite periodicity information and/or may be just too complicated for the periodicity information to be perceived by the analyst.